The concept of the invention relates to means for improving the quality of a signal, such as human speech, transmitted through a medium which causes non-linear distortion (i.e., a non-linear medium).
In the prior art of voice signal processing to overcome distortion due to the signalling medium, a variety of techniques have been employed. In the case of a frequency distorting signalling medium of known, fixed properties, the technique of the inverse filter has been used, whereby a network having a transfer function which is the inverse of the transfer function for the signalling medium (i.e., telephone line or whatever), is employed. In this way the a priori frequency distorted signal is re-distorted or compensatorily reshaped to its original waveform or shape. Such a technique is of limited effectiveness, however, under those circumstances where the properties of the signalling channel are unpredictable or variable. In such alternative case, the prior art has employed adaptive linear filter techniques. The most successful of these appear to be digital techniques applied to digitized data. Such digital techniques have been applied to overcoming linear distortions in a transmission line and for compensation of deviations from a linear response (described by a classical linear differential equation or difference equation whose coefficients are not functions of the amplitude of the input signal). Examples of such adaptive linear compensation techniques are disclosed in U.S. Pat. No. 3,524,169 to Gerald K. McAuliffe et al for IMPULSE RESPONSE CORRECTION SYSTEM, U.S. Pat. No. 3,573,624 to Jon P. Hartmann et al for IMPULSE RESPONSE CORRECTION SYSTEM and U.S. Pat. No. 3,614,623 to Gerald K. McAuliffe for ADAPTIVE SYSTEM FOR CORRECTION OF DISTORTION OF SIGNALS IN TRANSMISSION OF DIGITAL DATA.
Such digital adaptive filter or correlation techniques have also been useful in the extraction of noise lying within the signal spectrum of a noisy signal. Examples of correlation techniques for spectral equalization of electrical speech signals are taught in U.S. Pat. No. 4,000,369 to James E. Paul, Jr. for ANALOG SIGNAL CHANNEL EQUALIZATION WITH SIGNAL-IN-NOISE EMBODIMENT, and U.S. Pat. No. 4,052,559 to James E. Paul, et al for NOISE FILTERING DEVICE.
However, a limitation of such prior art techniques is the ineffectiveness of the device in correcting signals which have been subjected to a non-linear distorting process such as, for example, soft saturation.